I assume you are using the headphone out of the MP3 player and plugging it into the aux. input of the S900. If this is the case, you'll need to boost the signal in order to get line level volumes. You can do this with either a small mixer, or purchase an inexpensive headphone booster at Radio Shack for about $20. It's battery operated, very small and will boost the signal by about 200 percent.
PSR-S950, Bose L1 Compact, TC Helicon Harmony-M, Digitech VR, Samson Q7, Sennheiser E855, Custom Console, Morgan 33 Out Island Sailboat, and lots of other silly stuff!